1. Field of the Invention
The present invention relates to a control apparatus and a control method for a hybrid vehicle, and more particularly to a control apparatus and a control method for a hybrid vehicle mounted with an internal combustion engine and an electric motor serving as a source of power for causing the vehicle to travel.
2. Description of the Background Art
In view of environmental issues, hybrid vehicles traveling by efficiently using an internal combustion engine and an electric motor in combination have recently become practically available. Such a hybrid vehicle is mounted with a chargeable and dischargeable power storage unit, supplies electric power to the electric motor so as to generate drive force for starting or accelerating for example, and recovers electric power from the vehicle's kinetic energy in traveling on a downhill or braking for example.
For the hybrid vehicle of this type, a configuration has been proposed for charging the mounted power storage unit with electric power supplied from an external power supply such as commercial power supply. The vehicle having the power storage unit thus charged in advance by an external power supply can travel with the internal combustion engine kept stopped, as long as the vehicle travels over a relatively short distance, for commuting or shopping, for example. Accordingly, the fuel economy of the vehicle can be improved. The travel in such a manner is also referred to as “EV (Electric Vehicle) travel mode.”
Japanese Patent Laying-Open No. 09-098517 for example discloses how the state of charge of a battery changes relative to the travel distance, in a hybrid vehicle of the above-described type. According to the disclosure, before the vehicle starts traveling, a charger or the like is used to charge the battery (power storage unit) with electric power in advance, so that the state of charge of the battery is 100%. Then, the vehicle is caused to start traveling by driving an electric motor with the electric power stored in the battery. As the travel distance increases, the state of charge of the battery decreases. When the state of charge decreases for example to 50%, a power source (internal combustion engine) is driven so that an electric generator starts generating electric power. The electric power generated at this time is supplied to the electric motor and also stored in the battery. When the state of charge of the battery increases to reach 55%, the power source is stopped from being driven so that power generation by the electric generator is also stopped. Then, the vehicle travels by driving the electric motor with only the electric power of the battery again.
For the hybrid vehicle disclosed in Japanese Patent Laying-Open No. 09-098517 as described above, the travel distance in the EV travel mode can be increased by storing electric power in the power storage unit in advance before the vehicle is started.
Even while the vehicle is running in the EV travel mode, the internal combustion engine may be started if the driver of the vehicle makes a request for quick acceleration, for example, so that excessive discharge from the power storage unit can be avoided.
As long as the hybrid vehicle is mounted with the internal combustion engine, the hybrid vehicle is also mounted with a catalytic converter for purifying exhaust gas from the internal combustion engine, like the conventional vehicle having an internal combustion engine only as a source of power. Thus, a warm-up request may be given for promoting warm-up of the internal combustion engine or catalytic converter even while the vehicle is running in the EV travel mode. Then, even if the hybrid vehicle can run in the EV travel mode to reach the destination without driving the internal combustion engine, the travel distance could be difficult to extend since the internal combustion engine or catalytic converter is warmed up in response to the warm-up request, possibly resulting in deterioration in fuel economy.
Particularly for the hybrid vehicle of the type having the power storage unit chargeable with an external power supply as described above, the travel distance in the EV travel mode is increased and the vehicle may travel mostly in the EV travel mode. The problem as described above may be critical accordingly. Therefore, for the hybrid vehicle, the travel distance in the EV travel mode is considerably influenced by how the operating condition of the internal combustion engine is set.